Polyesters such as, for example, polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate, generally referred to as “polyalkylene terephthalates”, are a class of important industrial polymers. They are widely used in fibers, films, and molding applications.
Polyesters can be produced by transesterification of an ester such as dimethyl terephthalate (DMT) with a glycol followed by polycondensation or by direct esterification of an acid such as terephthalic acid (TPA) with a glycol followed by polycondensation. A catalyst is used to catalyze the esterification, transesterification and/or polycondensation.
For example, polyester can be produced by injecting a slurry mixture of TPA and glycol at about 80° C. into an esterifier. Linear oligomer with degree of polymerization less than 10 are formed in one or two esterifiers at temperatures from 240° C. to 290° C. The oligomer is then polymerized in one or two prepolymerizers and then a final polymerizer or finisher at temperatures from 250° C. to 300° C. TPA esterification is catalyzed by the carboxyl groups of the acid.
Antimony is often used for polymerization or polycondensation reaction. Three forms of antimony are widely used in commercial production, antimony oxide (Sb2O3), antimony glycolate, and antimony acetate. However, antimony forms insoluble antimony complexes that plug the spinnerets in fiber spinning and leads to frequent shutdowns to wipe spinnerets clean of precipitated antimony compounds. The antimony-based catalysts are also coming under increased environmental pressure and regulatory control, especially in food contact applications.
Titanium catalysts can be used in esterification, transesterification, and polycondensation reactions. For example, organic titanates, such as tetraisopropyl and tetra n-butyl titanates, are known to be effective polycondensation catalysts for producing polyalkylene terephthalates. However, the titanium catalysts tend to hydrolyze on contact with water forming glycol-insoluble oligomeric species, which lose catalytic activity. Polyesters produced from an organic titanate also generate yellow discoloration.
Titanium glycolate, formed from glycol and tetraalkyl titanate, has been shown to be useful as polycondensation catalyst. For example, JP57038817, SU602503, JP50016796, JP49057092, JP46003395, and JP45004051 disclose titanium glycolate solution for polyester polymerization. U.S. Pat. No. 3,121,109 also discloses the use of a titanium glycolate as catalyst in the presence of 2-20 parts of water per part of titanium. Both titanium glycolates and the water-containing titanium glycolate, however, react in the presence of air to form solids thereby becoming insoluble. See also JP07207010, U.S. Pat. No. 5,106,944, and U.S. Pat. No. 5,017,680 disclosing titanium catalyst solution produced from titanium tetraalkyl titanate and a salt. These titanium catalyst solutions stabilized by these metal salts are stable in air, but form solids or gel upon exposure to water and then air. Those solids can plug injection nozzle, pipe, or other process equipment in the polyester manufacturing process causing production interruptions.
Therefore, there is an increasing need for developing a catalyst that is stable in air, water, or both, is efficient, produces a polymer with reduced color, exhibits good catalytic activity, does not plug process equipment, and is environmentally friendly.
An advantage of the present invention catalyst composition is that the composition does not precipitate over prolonged storage or when used in process equipment, which contains air. Other advantages will become more apparent as the invention is more filly disclosed herein below.